1. Field of the Invention
The present invention relates to a process for preparing organic pentavalent phosphorus compounds, more particularly, a process for oxidizing organic trivalent phosphorus compounds using an aqueous hydrogen peroxide solution, without causing hydrolysis of the phosphorus compounds, to obtain the corresponding organic pentavalent phosphorus compounds.
2. Description of Related Art
The organic pentavalent phosphorus compounds typified by haloalkyl phosphate/polyphosphonates, trialkyl phosphates, triaryl phosphates, trialkyl phosphonates, triaryl phosphonates and trialkyl phosphinates are generally used as plasticizers, flame-retardants, stabilizers for synthetic resins, lubricants or metal-extracting reagents or the like. The organic pentavalent phosphorus compounds are usually obtained by oxidizing the corresponding organic trivalent phosphorous compounds (see Quat. Rev., 16, 208-239, 1962).
For example, the reaction of (n+1) moles of phosphorus trichloride with (2n+3) moles of an alkylene oxide followed by reacting with n moles of an aliphatic aldehyde or ketone is represented by the following scheme:
First Reaction ##STR1## Second Reaction ##STR2## wherein R and Z are, the same or different, a hydrogen atom or a lower alkyl group, and n is an integer from 0 to 10.
The obtained compound of the formula (I) is a (trivalent) phosphite/(pentavalent)polyphosphonate. The compound can be oxidized using various oxidizing agents to give a (pentavalent) phosphate/(pentavalent) polyphosphonate of the following formula (IV): ##STR3## wherein R and Z have the same meanings as defined for the formula (I).
The compound of the formula (I) consists of a structural part A of a trivalent phosphite and a structural part B of a pentavalent polyphosphonate. The structural part B having a P--C (phosphorus--carbon) bond can hardly be hydrolyzed and exhibits good heat resistance. On the other hand, the structural part A containing trivalent phosphorus can easily be hydrolyzed and is poor in heat resistance. Therefore, the compound of the formula (I) is hardly used as a flame retardant or a modifier for synthetic resins and polyurethane foams. In order to improve the hydrolytic resistance and heat resistance, trivalent phosphorus must be oxidized to pentavalent phosphorus.
A trivalent phosphorus compound represented by the formula (III): EQU (R.sup.1 O).sub.3-a P-R.sup.2.sub.a, (III)
wherein R.sup.1 and R.sup.2 are, the same or different, an alkyl or haloalkyl group having 1 to 18 carbon atom(s), or an aryl group having 6 to 10 carbon atoms, and a is an integer from 0 to 2,
consists of structural parts C and D as shown in the following formula when a=1, for example: ##STR4## wherein R.sup.1 and R.sup.2 have the same meanings as defined in the formula (III).
The structural part D of &gt;P--R.sup.2 has a &gt;P--C (phosphorus-carbon) band and is hardly hydrolyzed. On the other hand, the structural part C of (R.sup.1 O).sub.2 P-- is easily hydrolyzed. Accordingly, the compound of the formula (III) must also be stabilized by oxidation.
However, the aforesaid organic trivalent phosphorus compounds are hydrolyzed when oxidized only using an aqueous hydrogen peroxide solution as an oxidizing agent.
Oxidative methods of the organic trivalent phosphorus compounds are described in U.S. Pat. Nos. 3,027,395, 3,042,698, 3,042,700, and Japanese Unexamined Patent Publication No. Sho 51(1976)-6250 wherein propylene trimer hydroperoxide, sulfur, oxygen/ozone, and chlorine are used as an oxidizing agent, respectively. However, none of the above references disclose an oxidation method using an aqueous hydrogen peroxide solution as an oxidizing agent.